[Router] Add the shutdown-drain configuration surface (#39015)

Co-authored-by: Kangyan Zhou <kangyan.zhou@radixark.ai>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

experimental/sgl-router/tests/proxy/bucket_routing.rs

@@ -55,6 +55,7 @@ fn build_app_context(
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/chat_routing.rs

@@ -28,6 +28,7 @@ fn config_for(_worker_url: &str) -> Config {
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/common/cache_aware_fixture.rs

@@ -20,6 +20,7 @@ pub fn config() -> Config {
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/failover.rs

@@ -29,6 +29,7 @@ async fn failover_when_one_worker_dies() {
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: Default::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/graceful_shutdown.rs

@@ -38,6 +38,7 @@ fn build_ctx_with_worker(worker_url: &str) -> Arc<AppContext> {
         server: ServerConfig {
             host: "127.0.0.1".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/header_forwarding.rs

@@ -25,6 +25,7 @@ async fn forwards_whitelisted_headers_strips_others() {
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/pd_bootstrap_injection.rs

@@ -40,6 +40,7 @@ fn config() -> Config {
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/pd_pool_isolation.rs

@@ -39,6 +39,7 @@ fn config() -> Config {
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/pd_protocol_binding.rs

@@ -127,6 +127,7 @@ fn config() -> Config {
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/roundrobin_input_ids.rs

@@ -36,6 +36,7 @@ fn config() -> Config {
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/shared_prefill_admission.rs

@@ -143,6 +143,7 @@ fn config(policy: PolicyKind) -> Config {
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/sticky_input_ids.rs

@@ -48,6 +48,7 @@ fn config() -> Config {
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/sticky_routing.rs

@@ -36,6 +36,7 @@ fn build_sticky_ctx(header_name: &str, worker_urls: &[String]) -> Arc<AppContext
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {

experimental/sgl-router/tests/proxy/timeout.rs

@@ -32,6 +32,7 @@ fn config(_worker_url: &str) -> Config {
         server: ServerConfig {
             host: "0".into(),
             port: 0,
+            ..Default::default()
         },
         observability: ObservabilityConfig::default(),
         model: ModelConfig {